This invention relates to speedometers, and more particularly, to speedometers which utilize the Doppler Effect.
The Doppler Effect is a well known phenomenon. It occurs when a traveling object emits sound. When that happens, a stationary listener perceives the sound as being at a higher or lower pitch, depending upon whether the object is traveling towards or away from the listener. The Doppler Effect also occurs when the listener is moving and the source of the sound is stationary. In that situation, the pitch perceived by the listener depends upon the velocity of the listener relative to the sound source. The variation in pitch is governed by well known equations.
It is known in the art to use the Doppler Effect to measure speed. For example. U.S. Pat. No. 3,094,693, entitled "Velocity Sensing Apparatus." issued to D. G. Taylor, uses the Doppler Effect to measure the speed of a tractor. U.S. Pat. No. 4,065,745, entitled "Doppler Speedometer," issued to K. H. Robinson discloses the use of the Doppler Effect to measure the speed of a boat. Both these references disclose apparatus for providing a sound signal which strikes a surface, such as the ground, which reflects the sound back towards the apparatus. The apparatus receives the sound signal, and measures speed based on changes in the frequency spectrum of the reflected signal.